International Time Zones Key Terms & Points to Remember: Prime Meridian – the 0 line of longitude that passes through Greenwich in London. It runs from the North Pole to the South Pole and is called the Greenwich Meridian. Greenwich Mean Time (GMT) – What is it? When the sun is directly over the prime meridian, the time is 12 noon GMT. All other time zones are measured from this. International Date Line - the 180 line of longitude that runs from the North Pole to the South Pole on the opposite side of the globe to the Greenwich Meridian. What is the significance of the International Date Line? When travelling east, if you cross the International Date Line, you move into a new time zone and into the previous day on the calendar. Time Zones – the Earth is divided into time zones and each zone has a difference of approx 1 hour from Greenwich.